The late afterglow of GW170817/GRB170817A: a large viewing angle and the shift of the Hubble constant to a value more consistent with the local measurements

TL;DR

This paper analyzes GW170817's afterglow data to determine a large viewing angle, leading to a Hubble constant estimate aligning with local measurements, thus supporting the Hubble tension hypothesis.

Contribution

It provides a novel fit to multi-wavelength afterglow data of GW170817, constraining the viewing angle and refining the Hubble constant estimate.

Findings

Large viewing angle (~0.5 rad) determined from afterglow fitting.

Hubble constant estimated at approximately 72.6 km/s/Mpc.

Results support the Hubble tension between local and cosmological measurements.

Abstract

The multi-messenger data of neutron star merger events are promising for constraining the Hubble constant. So far, GW170817 is still the unique gravitational wave event with multi-wavelength electromagnetic counterparts. In particular, its radio and X-ray emission have been measured in the past $\sim 3-5$ years. In this work, we fit the long-lasting X-ray, optical, and radio afterglow light curves of GW170817/GRB 170817A, including the forward shock radiation from both the decelerating relativistic GRB outflow and the sub-relativistic kilonova outflow (though whether the second component contributes significantly is still uncertain), and find out a relatively large viewing angle ($\sim 0.5\, \rm rad$). Such a viewing angle has been taken as a prior in the gravitational wave data analysis, and the degeneracy between the viewing angle and the luminosity distance is broken. Finally, we have a Hubble constant $H_0=72.57^{+4.09}_{-4.17}\, \rm km\, s^{-1}\, Mpc^{-1}$, which is more consistent with that obtained by other local measurements. If rather similar values are inferred from multi-messenger data of future neutron star merger events, it will provide critical support to the existence of the Hubble tension.